This invention relates to apparatus for generating electrical waveforms. More particularly, it is concerned with apparatus for successively and repeatedly sampling a plurality of predetermined voltage levels to synthesize an analog waveform.
Analog voltage waveforms may be synthesized by sampling voltage levels which are produced at voltage nodes along a series string of resistances of predetermined values. The precision of the waveforms produced depends upon the precision of the voltage level at each node which, in turn, is determined by the precision of the resistance values between nodes along the series of resistances. Precise resistance values may be obtained with certain technologies. For example, with thin film technology techniques resistance elements can be trimmed by lasers to provide precise values.
In the fabrication of monolithic integrated circuits, specifically of the MOS type, however, it is difficult to obtain precise resistance values between nodes, as when the relationships are non-linear, for example. Typically, in MOS technology a series string of resistances is formed as a linear diffused region having taps at selected points along its length. Because of the size of the element and the physical constraints imposed by the processes employed, the taps require too much space to be considered as making contacts at points. That is, the taps themselves have an effect on the values of resistance between taps and thus on the precision of the voltage levels established at the voltage nodes.